1. Field of the Invention
The present invention relates to a method of manufacturing a p-n junction body of cubic boron nitride (hereinafter referred to as cBN), which is effectively applied to a diode, for example.
2. Description of the Background Art
cBN, which is synthesized under superhigh pressure, has a wide band gap of 6.6 eV, and is stable under a temperature up to 900.degree. C. in the atmospheric air. Further, a p-type of n-type semiconductor material can be obtained by doping cBN with an appropriate substance. Therefore, a cBN p-n junction body forming a semiconductor device prepared by joining p-type cBN with n-type cBN is expected to stably operate under a high temperature.
U.S. Pat. No. 3,078,232 (corresponding to the Japanese Patent Publication Gazette No. 7309/1963) discloses a p-type cBN doped by Be. U.S. Pat. No. 3,141,802 (corresponding to the Japanese Patent Publication Gazette No. 24856/1963) discloses an n-type cBN doped by Si and/or Ge wherein a p-n junction cBN grain is formed. U.S. Pat. No. 3,216,942 (corresponding to the Japanese Patent Publication Gazette No. 24857/1963) discloses an n-type cBN S and/or Se and a p-n junction cBN grain is formed. A conventional method of manufacturing a cBN p-n junction body is carried out as follows:
(1) Be is added to a mixture of boron nitride (hereinafter referred to as BN) source and a solvent material. Then the mixture is exposed to superhigh pressure and temperature conditions enabling formation of cBN, thereby to synthesize cBN having a p-type semiconductor property, the crystal lattice of which is partially replaced by Be.
(2) At least one of Si, Ge, S and Se is added to a mixture of a solvent material and BN. A p-type cBN is obtained in the item (1) is so disposed and exposed to superhigh pressure and temperature as to grow cBN having an n-type semiconductor property, the crystal lattice of which is partially replaced by Si, for example, on the p-type cBN. Thus, a cBN p-n junction body is obtained.
Another method of manufacturing a larger cBN junction body is proposed on pages 184-185 of "Program and Abstracts" of The 28th High Pressure Conference of Japan held at Kobe on Nov. 4-6, 1987. In this method, the so-called temperature gradient method is employed to grow a larger cBN crystal on a seed crystal by providing the seed crystal at the interface between BN source and a solvent material with a temperature relatively higher than that of the seed crystal part. In this case, seed crystals are prepared from p-type cBN and Si is added to the solvent, thereby to manufacture a cBN p-n junction body. The cBN p-n junction body obtained in the just mentioned method is larger in size and higher in quality than that obtained in the aforementioned method, since spontaneous nucleation and the growth rate of cBN can be controlled by adjusting the temperature gradient.
In the aforementioned method disclosed in U.S. Pat. Nos. 3,141,802 and 3,216,942, a large amount of cBN p-n junction bodies can be obtained at a low cost. However, each cBN p-n junction body thus obtained has extremely small size of about 300 .mu.m, to cause difficulty in handling in later steps. The size of the junction body is thus limited to about 300 .mu.m, since formation/growth of cBN is unavoidably caused by spontaneous nucleation from portions other than the seed crystals, thereby restricting the spaces allowing growth of cBN from the seed crystals.
On the other hand, a cBN p-n junction body having a size of 2 or 3 millimeters can be obtained in the method employing the temperature gradient method. However, this method requires an extremely long time of tens of hours, whereby the cost is inevitably increased.